The present invention relates in general to an improved rotary heat exchanger and in particular it relates to a novel heat exchange system for gas turbine engines.
Rotary heat exchangers or heat recovery wheels are utilized for the absorption of heat from exhaust gases travelling in one duct which heat is subsequently extracted from the wheel for preheating input gases, travelling in an adjacent duct. Rotary heat exchangers or heat recovery wheels are well known in gas turbine engines and in other applications.
Such heat recovery wheels are typically formed from glass-ceramic materials because of their low coefficient of thermal expansion, low thermal conductivity and their high resistance to oxidation and corrosion. In some applications it is desirable to drive such ceramic wheels from the periphery thereof by attaching a ring gear to the circumference of the wheel which mates with a pinion gear for rotating the wheel through the exhaust gas and input gas ducts. Because the aforementioned ring gear is typically made of metal which has a differing coefficient of thermal expansion from that of the ceramic, some provision must be made to accommodate the differences in the thermal expansion between the ceramic wheel body and the metal ring gear. One approach for providing such flexibility is disclosed in U.S. Pat. No. 3,623,544--McLean which discloses metal springs situated about the periphery of the ceramic heat exchanger and to which the aforementioned ring gear is attached. The metal springs are attached to the ceramic disk by means of solid glass-ceramic blocks cemented to the ceramic disk which surround and reinforce the springs. The McLean approach is expensive and a rather less expensive approach is disclosed in U.S. Pat. No. 3,525,384--Horton wherein, rather than by the provision of metal springs, the aforementioned ring gear is linked to the ceramic disk by means of an elastomeric material bonded to both components. Silicone elastomer materials have been used for this purpose, however, these tend to degrade rapidly above about 500.degree. F. (260.degree. C.). This degradation results in shrinking and hardening of the elastomeric material which may result in failure of either the bond of the elastomeric material to the ceramic disk or even failure of the ceramic material itself due to its inability to expand while constricted by the hardened elastomeric component. Since, in a typical 4 to 1 compression ratio gas turbine engine, the exhaust entering the rotary heat exchange often may reach temperatures of 1400.degree. F. (760.degree. C.), the aforementioned elastomeric material may rise to temperatures exceeding 500.degree. F. (260.degree. C.), which is above the degradation temperature of known elastomers.
It would be desirable to provide a rotary heat exchanger of the glass-ceramic type driven by a ring gear fastened to it by means of a flexible member which is of simpler construction and more inexpensive than the metal spring type members disclosed in the aforementioned McLean patent.
Moreover, it would be desirable to provide a rotary heat exchanger made from a glass-ceramic material and driven by a metal ring gear extending about the periphery thereof which is fastened to the heat recovery wheel by means of an elastomeric material and yet which does not suffer from the high temperature degradation mentioned above.